100 APPENDAGES TO THE MICROSCOPE. 



contrary kind must be taken. — Another instrument for the same 

 purpose is a flat Speculum of polished Steel, of smaller diameter 

 than the ordinary pupil of the eye, fixed at an angle of 45° in 

 front of the Eye-piece ; and this answers exactly the same end as 

 the preceding, since the rays from the eye-piece are reflected verti- 

 cally upwards to the central part of the pupil placed above the 

 mirror, whilst, as the eye also receives rays from the paper and 

 tracer, in the same direction, through the peripheral portion of the 

 pupil, the image formed by the Microscope is visually projected 

 downwards, as in the preceding case. This Disk, the invention of 

 the celebrated anatomist Soemmering, is preferred by some micro- 

 scopic delineators to the camera lucida. The fact is, however (as 

 the Author can testify from his own experience), that there is a 

 sort of ' knack ' in the use of each instrument, which is commonly 

 acquired by practice alone ; and that a person habituated to the use 

 of either of them does not at first work well with another. — A dif- 

 ferent plan is preferred by some Microscopists, which consists in 

 the substitution of a plate of neutral-tint or darkened Glass for the 

 oblique mirror ; the eye receiving at the same time the rays of the 

 microscopic image, which are obliquely reflected to it from the 

 surface of the glass, and those of the paper, tracing-point, &c, 

 which come to it through the glass. — In another very ingenious 

 arrangement, devised by Prof. Amici, and adapted to the hori- 

 zontal microscope by M. Chevalier, the eye looks through the 

 Microscope at the object (as in the ordinary view of it), instead of 

 looking at its projection upon the paper ; the image of the tracing- 

 point being projected upon the field, which is in many respects much 

 more advantageous. This is effected by combining a perforated 

 steel mirror with a reflecting prism; it is fitted to the Eye -piece 

 of the Microscope as shown in Fig. 53 ; and its action will be under- 

 stood by the accompanying diagram (Fig. 54). The ray a 6 proceeding 

 from the object, after emerging from the eye-piece of the Micro- 

 scope, passes through the central perforation in the oblique mirror 

 M which is placed in front of it, and so directly onwards to the 

 eye. On the other hand, the ray a' h' proceeding from the tracing- 

 point, enters the prism p, is reflected from its inclined surface to 

 the inclined surface of the mirror M, and is by it reflected to the 

 eye in such parallelism to the ray proceeding from the object, that 

 the two blend into one image. — The same effect is produced by a 

 contrivance which has been devised by MM. Nachet for use with 

 vertical Microscopes. It consists of a prism of a nearly rhom- 

 boidal form (Fig. 55), which is placed with one of its inclined 

 sides a c over the Eye-piece of the Microscope ; to this side is 

 cemented an oblique segment, e, of a small glass cylinder, which 

 presents to the ray a b proceeding directly upwards from the object 

 a surface at right angles to it ; so that this ray passes into the small 

 cylinder E, and out from the side A B of the larger prism, without 

 sustaining any refraction, and with very little loss by reflexion 



